In single engine light aircraft, it is common that a number of the instruments are vacuum or air pressure operated. These instruments include the heading indicator, attitude indicator and turn and bank indicator. Applicant's U.S. Pat. No. 4,364,268, issued Dec. 21, 1982 is directed to apparatus for providing a redundant vacuum or air pressure source to provide for continued operation of the instruments in the event there is a failure of the primary pump. Such apparatus for providing a backup vacuum or air pressure source includes a second pump connected to the instruments in such a manner that it becomes operable in response to failure of the primary pump. In normal practice the back-up pump is mounted in the engine compartment of the airplane forward of the fire wall.
Air pumps of the type commonly employed as vacuum or air pressure pumps used to operate aircraft instruments commonly comprise impeller type pumps having an aluminum housing and a carbon impeller. The pump is constructed such that the planar end faces of the impeller are very closely spaced with respect to opposed planar faces of the aluminum housing. The aluminum forming the housing has a coefficient of expansion of 0.00124 inch per 100.degree. F. The carbon material commonly used in the construction of an impeller commonly has a coefficient of expansion of 0.00025 inch per 100.degree. F. Because of the differences in the rates of expansion of these materials, if the pumps are placed in a very cold environment, for example, -40.degree. C., the aluminum housing may contract sufficiently that the impeller will be immovable and the pump inoperable.
When an airplane is operated at high altitudes, the ambient temperature may be -40.degree. C. or colder, and it is possible that the secondary pump for the aircraft instruments may become so cold that the pump will become inoperable.
Additionally, in the operation of vacuum operated instrument systems, when the backup pump is placed in operation, it draws air through the instruments being operated. These instruments are within the aircraft cabin, and the air flowing through the backup vacuum pump will be heated cabin air. At high altitudes or in very cold climates, this heated cabin air has a higher moisture content than the outside air. In the event the backup pump is at high altitude when it is called on, when the cabin air having high moisture content is pulled through the pump, the moisture may freeze on the interior surfaces of the pump and cause failure of the backup pump.